Battery is widely used for the power source in portable electronic products. However, the battery voltage will be gradually decayed with its operational time or suddenly dropped down resulted from instant increasing of load current flowing through the internal resistor of the battery. For a battery voltage will be out of a desired range, it is generally employed buck-boost converter or two-stage, i.e., boost-then-buck, voltage converter in order to maintain a stable output voltage for power supply to a load.
FIG. 1 shows a conventional two-stage voltage converter 10 that includes a boost converter 12 connected in series with a buck converter 14. The boost converter 12 is connected between a supply voltage VS provided by one or more batteries and an output 1202 to boost up the supply voltage VS to generate an output voltage VOUT1 to supply for a load 162 connected to the output 1202, and the buck converter 14 is connected between the output 1202 and 1402 to convert the boosted voltage VOUT1 to another output voltage VOUT2 to supply for another load 164 connected to the output 1402. For typical applications, the supply voltage VS is in the range of from 1.8V to 3.3V, the boosted voltage VOUT1 is about 3.3V, and the bucked voltage VOUT2 is about 1.8V. The boost converter 12 comprises an inductor L1 connected between the supply voltage VS and a node 1204, a diode D1 connected between the node 1204 and the output 1202, a transistor Q1 connected between the node 1204 and ground, a capacitor C1 connected between the output 1202 and ground, and a boost controller 122 to switch the transistor Q1 for regulating the output voltage VOUT1. On the other hand, the buck converter 14 comprises an inductor L2 connected between the output 1402 and a node 1404, a diode D2 connected between the node 1404 and ground, a capacitor C2 connected between the output 1402 and ground, a transistor Q2 connected between the output 1202 and the node 1404, and a buck controller 142 to switch the transistor Q2 for regulating the output voltage VOUT2. However, for the two-stage voltage converter 10 boosting up the supply voltage VS first and then bucking down the boosted voltage VOUT1, the total efficiency to convert the supply voltage Vs to the output voltage VOUT2 will be the efficiency product of the boost converter 12 and the buck converter 14, i.e., ηBoost×ηBuck, and therefore, the total efficiency of the two-stage voltage converter 10 is decreased by such two-stage conversion.
FIG. 2 shows a conventional SEPIC converter 20 that comprises a boost converter 22 and a buck-boost converter 24 both connected to a supply voltage VS. As usual, the boost converter 22 is connected between the supply voltage VS and a load 262 connected to its output 2204, to boost up the supply voltage VS to generate an output voltage VOUT1, at the output 2204. The buck-boost converter 24 is connected between the supply voltage VS and another load 264 connected to its output 2406, to convert the supply voltage VS to another output voltage VOUT2 at the output 2406. The boost converter 22 comprises an inductor L1 connected between the supply voltage VS and a node 2202, a diode D1 connected between the node 2202 and the output 2204, a capacitor C1 connected between the output 2204 and ground, a transistor Q1 connected between the node 2202 and ground, and a boost controller 222 to switch the transistor Q1 for regulating the output voltage VOUT1. On the other hand, the buck-boost converter 24 comprises an inductor L2 connected between the supply voltage VS and a node 2402, another inductor L3 connected between a node 2404 and ground, a diode D2 connected between the node 2404 and the output 2406, a capacitor C2 connected between the output 2406 and ground, another capacitor C3 connected between the nodes 2402 and 2404, a transistor Q2 connected between the node 2402 and ground, and a buck controller 242 to switch the transistor Q2 for regulating the output voltage VOUT2. However, a buck-boost converter does not have high conversion efficiency, and the two energy-storing elements, inductors L2 and L3, bring the buck-boost converter 24 to high cost and large size.
Moreover, as shown in FIG. 1 and FIG. 2, other transient loadings 160 and 260, such as photoflash and motor, also connected to the supply voltage VS would generate surge current It that causes the supply voltage VS suddenly dropped down because of the surge current It flowing through the internal resistor of the battery, and thereby the supply voltage VS may be lower than the output voltage VOUT2, as shown by curve 406 in FIG. 4, to further degrade the efficiency thereof.
Although both the voltage converters 10 and 20 shown in FIG. 1 and FIG. 2 may maintain the output voltage VOUT2 stably at desired level, their conversion efficiencies are only around 80%, as shown in FIG. 5 by curve 52 for the two-stage voltage converter 10 and by curve 54 for the SEPIC converter 20.
Therefore, it is desired an efficiency improved voltage converter.